Crude plant extracts have been found to cause an increase in the number of delta cells of the anterior pituitary and the number of developing follicles in the ovaries of Microtus montanus. Early onset of sexual maturity, increased litter size, greater frequency of post- partum estrus and general hypertrophy of the reproductive organs also result from dietary supplements of fresh green plants or plant extracts. Various extraction, column separation and spectroscopic techniques will be employed to chemically identify the active plant factor that stimulates reproductive activity in the meadow vole, M. montanus. Throughout the identification process the active compound will be verified using a bioassay for uterine hypertrophy in M. montanus. Having defined the chemical nature of the plant factor, studies will be undertaken to determine the physiological mode and site of action of the active compound.